Add a whiteboard file picker and force-refresh

File picker: an optional WebDAV browse root in Settings
(User.webdav_base_url) plus a plain-PROPFIND directory listing
(webdav_client.list_directory) power a "Browse..." panel on a frame's
Whiteboard tab, so a file can be clicked into rather than typing its
exact WebDAV URL. Manual URL entry still works unchanged either way.

Force-refresh: get_or_refresh_whiteboard takes a force flag that skips
the fetch throttle entirely; the Whiteboard tab's refresh button now
passes it, so clicking it always re-fetches and re-renders instead of
possibly just re-showing the same cached image from within the last
~20 minutes.

server/app/templates/settings.html

@@ -93,6 +93,15 @@
                  placeholder="{% if user.webdav_password %}(unchanged -- enter a new password to replace){% else %}app password or account password{% endif %}">
         </label>
       </div>
+      <label>WebDAV browse root (optional)
+        <input type="text" id="webdav_base_url" name="webdav_base_url" autocomplete="off"
+               placeholder="https://cloud.example.com/remote.php/dav/files/you/"
+               value="{{ user.webdav_base_url }}">
+      </label>
+      <p class="sub" style="margin-top: 4px;">A starting folder for the
+        file picker on a frame's Whiteboard tab, so you can browse to a
+        file instead of typing its exact URL. Not required -- you can
+        still paste a URL directly without setting this.</p>
       <p class="sub" style="margin-top: 8px;">Doesn't show up anywhere by
         itself -- point a specific frame's Whiteboard tab at a file URL
         using this account.</p>